Resonance-type effects in free radical electron spin–lattice relaxation and electron spin echo dephasing due to a dynamics of a homogeneous-chain oligomeric system

Abstract

Electron spin–lattice relaxation and phase relaxation were measured by the electron spin echo (ESE) method in the temperature range 4.2–300 K for radicals appearing during the polycondensation reaction of poly(4-hydrazo-diphenylene disulfide) oligomers with N-methyl-2-pyrrolidone chain ends. The spin–lattice relaxation rate 1/T₁ is governed by cross-relaxation to the fast relaxing radical pairs, with the relaxation rate rapidly increasing at low temperature and showing unusually weakly temperature dependent behaviour above 25 K. At 146 K the resonance peak from rotating CH₃ groups (τ₀ = 1.07 × 10⁻¹² s, E_a = 441 cm⁻¹) appears. Such a resonance-type enhancement has been not observed so far in electron spin–lattice relaxation in solid-state materials. The ESE dephasing rate 1/T_M is temperature dependent with a pseudo-resonance minimum at about 100 K resulting from two different dephasing mechanisms. At low temperatures the dephasing rate decreases on heating due to quantum tunnelling motion of CH₃ groups between nearly equivalent potential wells. Above 100 K the rate continuously increases with temperature as a result of thermally activated low frequency motions between shallow potential minima (τ₀ = 2.6 × 10⁻⁷ s, E_a = 49 cm⁻¹). Various processes leading to temperature variations of the dephasing rate are reviewed and discussed. The relaxation results are compared with those for radicals spontaneously occurring in partially cured phenol-formaldehyde resin.